The Gregorian calendar recognizes two eras: B.C. (before Christ) or B.C.E. (before common era), and A.D. (Latin "Anno Domini", which means "in the year of the Lord") or C.E. (common era). This implementation of the GregorianCalendar class recognizes only the current era (A.D. or C.E.).

A leap year in the Gregorian calendar is defined as a year that is evenly divisible by four, except if it is divisible by 100; however, years that are divisible by 400 are leap years. For example, the year 1900 was not a leap year, but the year 2000 was. A common year has 365 days and a leap year has 366 days.

The Gregorian calendar has 12 months with 28 to 31 days each: January (31 days), February (28 or 29 days), March (31 days), April (30 days), May (31 days), June (30 days), July (31 days), August (31 days), September (30 days), October (31 days), November (30 days), and December (31 days). February has 29 days during leap years and 28 during common years.

GetEra ignores punctuation in abbreviated era names, only if the GregorianCalendar is selected in DateTimeFormatInfo.Calendar and the culture uses "A.D." as the era name; that is, "A.D." is equivalent to "AD".